there are a number of solutions to this problem but the one I have found to be most efficacious over ther years which I always teach my students is to shout 'woof!' as loud as possible as you are playing the note.
not only does this cover up the offending sound but it clears the room of any stray cats that may not belong to you, thus protecting you from a slew of law suits ranging from cat napping to 'nefarious acts with animals while crossing a bar line,'
Hope this helps,